From SABR member Ben Lindbergh at Baseball Prospectus on May 30, 2013:

When I got home from the SABR Analytics Conference in mid-March, I spent a week or so writingandtalking about some of the most interesting things I’d heard there. But there was one particularly intriguing topic that I wasn’t yet ready to write about.

That topic was brought up by Bill James, who's given us more than his fair share of interesting insights. Here’s all the backstory you need to know: James was on an “Analytics Super Panel” with Brian Kenny and Joe Posnanski, and he was asked by an audience member whether there’s any utility for teams in gathering information on home plate umpires.

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One thing that, to answer your question, the different first base, third base umpires are wildly inconsistent in how they call the checked swing strike. Major league teams are aware of that, and under certain conditions, the starting pitcher will be aware that that first base umpire likes to call, will call the strike, because if you throw a slider down and away, he’ll start to swing at it, he’ll check, there’s a swing, and it’s really important that that’s an umpire that tends to call that a strike or tends not to call it a strike. So, yeah, we’re aware of that stuff, and we watch it, and I would bet almost every team is on top of that issue.

We all know that home plate umpires have different strike zones, and we’re aware that teams pay attention to and try to exploit that information. (Here’s a recent picture of some of Inside Edge’s umpire charts taped up in the Dodgers’ dugout.) But before I heard James say it, I’d never considered that checked-swing strike rates might vary by base ump, or that pitchers might alter their approach accordingly. Of course, there’s no precise definition of what constitutes a swing in the official rulebook, so it makes sense that certain umpires would see swings differently. Some might rule based on whether the batter’s wrists “break,” or whether the barrel of the bat passes in front of the plate. Others might try to assess the batter’s “intent to swing.”